1. Field of the Invention
This invention relates to ohmic contacts used on n-type GaAs and GaAlAs devices.
2. Discussion of Prior Art
GaAs and GaAlAs are preferred materials for the manufacture of high speed devices such as bipolar transistors, heterojunction bipolar transistors or high mobility electron transistors. However, for efficient use of the higher device operating speeds of such materials, it is necessary to minimise the resistance of the ohmic contacts. Lower ohmic contact resistances lead to a reduction in undesirable heat production, improved frequency response and an increase in noise immunity for logic circuits.
Alloyed Ge-Au and Ni-Ge-Au films are widely used for fabricating ohmic contacts to n-type GaAs. In general, adding small amounts of Ni to Ge-Au leads to lower contact resistance as well as serving to maintain a smooth surface morphology after alloying the contact metallisation.
Heime et al (Sol. St. Electronics 17 pp 835-837 1974) have reported specific contact resistivities of 4.times.10.sup.-4 .OMEGA.cm.sup.2 for carrier concentrations of 2.times.10.sup.15 cm.sup.-3 and 1.times.10.sup.-6 .OMEGA.cm.sup.2 for carrier concentrations of 1.times.10.sup.18 cm.sup.-3. These values were achieved using contacts where a 50 A layer of Ni was evaporated onto GaAs, with a 700 A layer of eutectic AuGe laid on the nickel and a final 300 A Ni overplate evaporated onto the AuGe.
In 1987 Rai et al (J. App. Phys. 61(9) pp 4682-4688 1987) were able to achieve an ohmic contact resistance minimum of about 0.2 .OMEGA.-mm after an alloy anneal of 420.degree. C. Their ohmic contact metallised structure consisted of sequential electron beam deposited layers of 50 A Ni; 170 A Ge; 300 A Au; 150 A Ni; and 2000 A Au. This contact definition was deposited on n.sup.+ GaAs. A direct comparison of this reported ohmic contact resistance with that of the contacts reported by Heime et al is difficult due to the fact that Heime et al did not report the sheet resistance of the substrate material.
Further reductions in ohmic contact resistance are reported in J. Vac.. Sci. Technol. B4(4) pp 903-911 1986 by Murkami et al. Three different ohmic contact constructions were detailed, each deposited on GaAs implanted to contain 1.times.10.sup.18 cm.sup.-3 carriers. The mean contact resistance of each type of contact construction was about 0.1-mm or slightly greater. Type A sample was prepared by sequentially depositing a 1000 .ANG. Au-Ge layer followed by 350 .ANG. Ni and 500 .ANG. Au layers. Sample types B and C were prepared by depositing 50 and 100 .ANG. of Ni, respectively, as the first layer, followed by Au-Ge (1000 .ANG. thick), Ni (300 .ANG. and 250 .ANG. thick respectively) and Au (500 .ANG. thick) layers.
Ohmic contacts may also be made to n-type GaAlAs layers by the use of a thin buffer GaAs contact layer. The buffer layer prevents oxidation of Al and also helps to improve adhesion.